Diabetes mellitus is a disorder accompanied by a persistent hyperglycemia, and is said to result from interaction between a variety between environmental factors and genetic factors. Blood sugar is mainly controlled by insulin, and elevated blood sugar is known to result from insulin deficiency or excess of factors (such as genetic disposition, lack of exercise, obesity, and stress) that inhibit the action of insulin. Diabetes mellitus is divided into two types: type I diabetes mellitus caused by the reduced function of insulin secretion due to autoimmune disease, etc., and type II diabetes mellitus caused by the reduced function of pancreatic insulin secretion due to pancreatic fatigue associated with persistently elevated insulin secretion and insulin resistance. In Japan, diabetes mellitus is a national affliction, and more than 95% of the diabetic patients (predicted to exceed 20 million people including diabetic candidates) are said to be non-insulin-dependent diabetes mellitus. The increasing number of patients due to changes in life style is posing a problem. On a global level, the number is expected to be about 200 million (Non-patent document 1), and the global market size of antidiabetics is about one trillion yen, indicating that diabetes mellitus is the number one disease in terms of the market size of its therapeutic agents and the number of population.
The treatment of diabetes mellitus, in mild cases, mainly comprises diet therapy, exercise therapy, amelioration of obesity, etc., and, in more advanced cases, the administration of oral antidiabetics (insulin secretion-promoting drugs) and, in further severe cases, the administration of insulin preparations. As insulin secretion-promoting substances (or insulin secretion-promoting drugs), there are known, in addition to glucose, amino acids (specifically arginine), β-receptor stimulating agents, α-receptor blockers, sulfonyl urea drugs and the like. Among them, sulfonyl urea drugs stimulate pancreatic β cells to promote intrinsic insulin secretion, but the timing and the amount secreted of insulin depend not on blood sugar level but on the timing and dosage of the drug. Thus, as a side effect, hypoglycemia may occur due to the persistent action of the drug (Non-patent document 2). Thus, conventional insulin secretion-promoting drugs and insulin preparations had the above problems. There have been needs therefore for drugs that permit more precise blood sugar control, i.e. drugs that not only decrease blood sugar but can maintain blood sugar in the normal range. For this purpose, simple screening methods are needed in order to obtain substances that are useful as insulin secretion-promoting drugs and insulin preparations.
L-arginine, an amino acid, plays a variety of roles in vivo. For example, it is involved in the control of immunological function, wound healing, hormone secretion, vascular tone, endothelial function, and insulin secretion (Non-patent document 3). L-arginine is a biologically active food-borne compound that mediates a variety of physiological actions. In this connection, a metabolic enzyme that uses L-arginine as a substrate is L-arginine interacting factor (AIF). However, little is known about AIF that directly mediates the physiological actions of L-arginine. The present inventors have so far succeeded in the purification and identification of AIF using L-arginine methyl ester (AME)-immobilized magnetic nanobeads developed by us (Non-patent document 4). While AME, similarly to arginine, promotes insulin secretion as an arginine agonist on the one hand, it inhibits nitrogen monooxide synthase (NOS) as an arginine antagonist similarly to L-nitro-L-arginine-methylester) on the other. Using this AME-immobilized magnetic nanobeads, phosphofructokinase (PFK), RuvB-like 2 (RBL2), and RuvB-like 1 (RBL1) were identified as AIF (Non-patent document 4).
A “protein comprising an amino acid sequence represented by SEQ ID NO: 2” used in the screening method of the present invention is a newly identified insulin secretion regulation factor that was isolated using the above AME-immobilized magnetic nanobeads described in Non-patent document 4. The amino acid sequence per se represented by SEQ ID NO: 2 has been registered to GenBank as NM—198899. However, there have been no reports so far that a protein consisting of an amino acid sequence represented by SEQ ID NO: 2 and a nucleic acid consisting of the base sequence encoding said amino acid sequence described in the present specification are involved in the promotion of insulin secretion.